1. Field of the Invention
This invention pertains to general arrangement of hydrofoils on a boat, the effect the arrangement has upon control and stability in the heel and pitch directions, the retraction of hydrofoils and the consolidation of the arrangement within a restricted envelope.
2. Description of Related Art
The main stream of sailing hydrofoil development has been directed toward heel control by wide spread principal load carrying hydrofoils or arranging the foils such that the load vectors from the foils pass through the intersection of the weight and sail load vectors, a so called compensated heel loading wherein little or no heel moment is experienced from sail loads. These designs resulted in sailing hydrofoil configurations with overall beam widths in excess of hull length, an arrangement which causes difficulty in tacking and docking, creates an appearance unacceptable by the consumer and causes wind blown spray from the windward foil to soak the crew.
Another difficulty in the main stream development has been the lack of sufficient pitch stability to handle wind gusts and rough water. The height control of these sailing machines depends on the area change of surface penetrating foils, which is insufficient to cause a forceful and immediate response to change in depth of the foil penetration. Ventilation of surface penetration type foils aggravates the stability problem. For the above reasons sailing hydrofoils have never been a commercial success.
The development of this invention is a result of sailing the experimental model depicted in U.S. Pat. No. 4,027,614. This model provided a number of combinations of hydrofoil arrangements. The one most successful but not shown was with the two mainfoils extending inboard with all other foils except the stabilizers retracted. This arrangement provided no heel stabilization from the main foils but the crew weight on the windward side provided sufficient stabilization for the sail loads required to execute a takeoff. As speed quickly increased after takeoff, the rudder mounted stern hydrofoils became more efficent in providing the major effort in heel control. This configuration was later improved by combining the two inboard foils into one continuous "V" shaped main hydrofoil mounted between the hulls enabling retraction forward to a position above the flotation waterline. This arrangement controlled height of the boat by area change of the "V" foil but proved to be marginal in pitch stabilization and required manual pitch attitude trimming of the boat. U.S. Pat. No. 4,517,912 describes the further improvement made to eliminate ventilation problems and provide a successful pitch stabilization system.
Computerized dynamic stability programs were written to simulate the experimental prototype in flight situations requiring stability, for instance, recovery from a severe gust. This program revealed that a forceful opposing reaction to vertical motion is required of the main hydrofoil in preventing a dive into the water as a result of a severe gust. This was corroborated by comparing operation of the fixed hydrofoil equipped prototype with the controlled angle of attack equipped modification.
Performance programs prepared utilizing refined drag data for each drag inducing component have reported surprisingly good performance which has been corroborated by sailing performance tests. The gains made in automatic or manual control of heel, automatic control of pitch attitude and height of the boat, elimination of ventilation problems and an impressive improvement in pitch stability has been well worth the price of minor sacrifices in performance at cruising speeds which in turn have been offset by greater allowable maximum speeds. Windward performance has been improved by the use of catamaran configuration. Sailing to windward involves a combination of flotation and foil support. With one hull in flotation, heel resistance is improved providing better windward vector component speeds than sailing more off the wind on the foils. However, on the foils actual boat speeds are superior.